The present invention relates to sintered bodies produced by a powder metallurgical process. In particular, this invention is concerned with sintered bodies such as sintered Ti bodies, sintered magnetic bodies of the Fe-Si type having excellent alternating current (a.c.) characteristics, and their production process.
Prior to this application, one or more of the inventors of this invention and others already have filed "Sintered Fe-Co Type Magnetic Materials and Production Process Thereof (PCT JP89/00537)" and "Stainless Steel Sintered Materials and Production Process Thereof (PCT JP89/00633)".
In these applications, the inventors have described a superior sintered Fe-Co magnetic body which has low C and O contents and high density, a sintered stainless steel body which has high density and high corrosion resistance, and the method of production of these sintered bodies.
Ti has a smaller specific gravity as compared to steel material but its strength is by no means inferior to steel, so that it is employed as a material for a variety of aircraft parts. It is also used for such medical purposes as prostheses and for orthopaedic surgery because it has good compatibility with human tissues, and is without deleterious effects to the human body.
Ti parts have heretofore been formed from Ti ingots. These conventional Ti parts however incur a high production cost and their productivity is low, since their machining yield is low. It has been known that the use of a powder metallurgical process makes it possible to produce sintered Ti bodies at a low cost and with high productivity. Ti is however a very active metal so that the surfaces of its particles tend to be covered with an oxide. This has led to the problem that high-density and low-impurity bodies are difficult to obtain regardless what conventional sintering process is employed. Further, any attempt at using a high-purity Ti powder of low C and O contents inevitably results in a high material cost, whereby the inherent economy of powder metallurgy is impaired.
Injection molding requires a binder in addition to a powdered raw material. It is however difficult to completely remove the binder in a subsequent step, so that the resulting sintered body has high C and O contents. No sintered high-density bodies can therefore be obtained.
Hot isostatic pressing is also recognized as a process for the production of sintered high-density bodies. This process however requires a complex and expensive apparatus and is hence accompanied by the problems of requiring a longer working time and being economically disadvantageous.
Y. Kaneko et al. at Faculty of Science and Engineering, Ritsumeikan Univ. have reported on the injection molding of titanium powder (Abstracts of Papers, Spring Meeting of Japan Society of Powder and Powder Metallurgy, 1988, pp 126 to 127). According to the abstract, a sintered body having a density ratio of 92% was obtained from Ti powder, by injection-molding the powder together with a binder and sintering the molded body at 1300.degree. C. for two hours under a reduced pressure of 0.01 Torr. In addition, it is obvious that this sintered body has a fairly high oxygen content, because the abstract also describes formation of TiO.sub.2 based on the results of X-ray diffraction analysis and microscopic observation of the structure.
On the other hand, Fe-Si alloys feature high electrical resistivity among soft magnetic materials. Assisted further by their low core losses, they are employed widely for a.c. applications.
There is however a limitation imposed on the application of sintered bodies of such Fe-Si alloys due to the low compressibility inherited from the hard and brittle properties of the alloys. This tendency is especially remarkable in the case of Fe-Si alloys which contain Si in a proportion of about 3 wt. % or higher.
"Sintering Behavior, Mechanical and Magnetic Properties of Sintered Fe-Si Materials" have been described in the International Journal of Powder Metallurgy & Powder Technology Vol. 20, No. 4, 1984. In this report, the authors described "Fe-Si (sintered) materials that were prepared by varying both the starting Fe powder and the way of Si addition. Water atomized iron powder mixed with pre-alloyed FeSi30 proved to be the most successful."
As a method for improving moldability, injection molding using an organic binder is regarded as a promising candidate because the hardness of a powder is practically immaterial. But when a metal powder is shaped by injection molding and then sintered, there is no method known for eliminating C, which is derived from the organic binder, without extreme oxidation of a highly oxidative element such as Si. It has hence been impossible to provide a sintered body excellent in a.c. magnetic characteristics.